Vibronic sensors are often used in process and/or automation technology. In the case of fill-level measuring devices, they have at least one mechanical oscillating unit—for example, a vibrating fork, a rod, or a membrane. During operation, the latter is excited into mechanical oscillation by a driving/receiving unit, often in the form of an electromechanical transducer that can, in turn, be a piezoelectric drive—for example, or an electromagnetic drive. The mechanical oscillating unit can, however, also be designed as an oscillating pipe in the case of flow meters, through which the respective medium flows, such as in a measuring device operating under the Coriolis principle.
A wide variety of corresponding field devices are made by the applicant and, in the case of fill-level measuring devices, are distributed under the name LIQUIPHANT or SOLIPHANT. The underlying measurement principles are known in principle from numerous publications. The driving/receiving unit excites the mechanically oscillating unit to mechanical oscillation via an electrical excitation signal. Conversely, the driving/receiving unit can receive the mechanical oscillations of the mechanical oscillating unit and transform them into an electrical receiving signal. The driving/receiving unit is accordingly either a separate driving unit and a separate receiving unit, or a combination driving/receiving unit.
Both the excitation signal and the receiving signal are characterized by their frequency, amplitude, and/or phase. Changes in these variables are, therefore, usually used for determination of the appropriate process variables, such as a pre-determined fill-level of a medium in a container or the density and/or viscosity of a medium, or the flow of a medium through a pipe. In the case of a vibronic limit level switch for liquids, for example, a distinction is made as to whether the oscillating unit is covered with the fluid or vibrates freely. The two states, the free state and the covered state, are thus differentiated—for example, based upon different resonant frequencies, i.e., a frequency shift. The density and/or viscosity, in turn, can be gauged using such a measuring device only if the oscillating unit is covered by the medium.
The driving/receiving unit is usually part of a feedback electrical oscillation circuit by which the excitation of the mechanical oscillating unit to mechanical oscillations is accomplished. A specifiable value for the excitation is frequently set via a control circuit for the phase shift, i.e., a target value for the phase shift between the excitation signal and the receiving signal. For example, an amplification factor of ≥1 and oscillating circuit conditions according to which all phases occurring in the oscillating circuit result in a multiple of 360° must be met for a resonant oscillation. A variety of methods for stimulating a mechanically oscillating unit or for setting a specifiable phase shift are known from the prior art. A basic distinction can be made here between an analog and a digital excitation, wherein the difference is between the oscillating circuit, made up of analog components that must be adapted to the type of sensor used, and digital methods, which are, in principle, universally applicable.
According to a frequently used excitation principle, the control circuit includes an amplifier and a phase shifter, by means of which the receiving signal is coupled back to the transmission signal in order to set the specifiable value for the phase shift between the excitation signal and the receiving signal. According to German patent, DE102006034105A1, for example, an adjustable phase shifter is used. The phase shifter is controlled via a control unit that measures the frequency of the previously amplified receiving signal and is at least based upon stored data on the frequency phase dependence of an amplifier unit.
An amplifier is also known from German patent, DE102007013557A1 that has an adjustable amplification factor, which is set by a control unit in such a manner that the amplitude of the transmission signal is generally located within a specifiable amplitude band.
A vibronic sensor is known from German patent, DE102005015547A1, wherein the electronics unit is provided with at least one all-pass filter for setting a target value for the phase shift. The all-pass filter changes the phase of an electrical signal at a constant gain as a function of the frequency. The all-pass filter can especially be controlled or regulated in such a manner that the phase between the excitation signal and the receiving signal is adjustable. According to one embodiment of the invention, the receiving signal is preferably only filtered and/or amplified before it is supplied to the all-pass filter, processed by it, and returned.
In the case of an analog excitation, however, the analog components, from which the oscillation circuit is built, must necessarily be adapted to the type of sensor used. The robustness of the sensor, especially with regard to external vibrations, is further dependent upon the selectivity of the filter used in each case for signal processing or evaluation, wherein the filters used determine the phase response of the electronics unit. The greater the pitch of the phase response, the narrower the frequency range to be covered by the filter is. Accordingly, there can be circumstances in which the sensor no longer oscillates in resonance.
Another excitation method is described in German patent, DE102009026685A1. The mechanically oscillating unit is successively excited to mechanical oscillations via consecutive discrete excitation frequencies by means of a so-called frequency sweep within a pre-determined frequency band in the operating range of the oscillating unit, and the corresponding receiving signals are recorded. Via the frequency sweep, that excitation frequency is determined at which the oscillating unit oscillates at an oscillating frequency that corresponds to a specifiable value for the phase shift. This excitation frequency is applied in each case to the oscillating unit. An advantageous development of this method is the subject matter of German patent, DE102009028022A1, in which the evaluation of the received signal is simplified by the receiving signal being sampled and evaluated phase-selectively only at specific instants. Similarly, it is proposed in German patent, DE102010030982A1 that the receiving signal be sampled at pre-determined discrete instants in relation to the transmission signal, the sampled voltage values of the receiving signal be each compared to the target value, which the receiving signal at this instant assumes if the specifiable value for the phase shift is present, and, in the case of deviation of the voltage value from the target value, the frequency of the transmission signal be decreased or increased based upon whether the deviation is positive or negative.
In the case of an excitation via a frequency sweep and the evaluation of the respective phases and/or amplitudes of the receiving signal, it must also be noted that there is a dependency between the flow speed of the frequency sweep and the frequency resolution.
An additional digital possibility for a vibronic sensor to regulate the phase shift between an excitation signal and a receiving signal is disclosed in DE00102010030982A1. The method described there is based upon the functional principle of a phase-locked loop, PLL. The frequency of the excitation signal is set here in such a manner that there is a specifiable value for the phase shift between the excitation signal and the receiving signal.
This type of excitation has decisive advantages with respect to evaluation speed, compared to excitation via a frequency sweep. To be sure, however, at least one phase detector is required, which influences robustness, i.e., especially, the stability of the control, among other things, if external vibrations occur, as well as the precision of the control circuit. In order for evaluation to be done in a stable manner, it must additionally be ensured that the amplitude of the excitation signal is kept at a constant value.
In order to reduce problems from the occurrence of external vibrations during the operation of a vibronic sensor, such as vibrations from pumps or ultrasonic baths, German patent, DE102012101667A1 proposes configuring a control/evaluation unit in such a way as to control the oscillation excitation in the presence of at least one external vibration as a function of the frequency and/or amplitude of the external vibrations, so that the receiving signal is essentially not disturbed by the external vibration, and/or to suppress at least one frequency of an external vibration.